Advancements in Cryptographic Protocols

At Eurocrypt 2024, Georg Fuchsbauer delivered a presentation on his work with Mathias Wolf titled “Concurrently Secure Blind Schnorr Signatures.” Their research addresses a critical need for blind signatures in various applications, particularly within blockchain systems, where compatibility with existing signature schemes is essential. The team developed a concurrently secure blind-signing protocol for Schnorr signatures, which are becoming standardized and widely supported by major cryptocurrencies. Their protocol thwarts denial-of-service attacks by leveraging standard cryptographic primitives such as Non-Interactive Zero-Knowledge proofs (NIZK) and Public Key Encryption (PKE), assuming the unforgeability of Schnorr signatures.

This protocol is the first to maintain compatibility with standard Schnorr implementations over 256-bit elliptic curves, a significant advancement for blockchain and cryptocurrency systems. Fuchsbauer and Wolf also introduced the novel concept of predicate blind signatures, allowing signers to define conditions that the blindly signed messages must meet. The researchers provided practical implementations and benchmarks for various use cases, including the example of blindly signing Bitcoin transactions only when they satisfy specific conditions defined by the signer, showcasing the real-world potential of their protocol.

References: Paper, Slides of the talk, Video

Georg Fuchsbauer also contributed to the research on “Updatable Public Key Encryption, Revisited,” co-authored with Joël Alwen and Marta Mularczyk. The team revisited the concept of Updatable Public Key Encryption (UPKE), originally introduced as a practical solution for building forward-secure cryptographic protocols. They observed that existing UPKE notions lack the syntactical flexibility and security required for many of the multi-party protocols that initially motivated UPKE’s development. In response, they provided a comprehensive taxonomy of UPKE properties—some of which had been overlooked in previous work—and offered a thorough overview of known UPKE constructions.

The presentation also introduced a new formal definition for UPKE, capturing the key properties necessary for secure multi-party protocols. The team proposed a practical pairing-based UPKE construction that offers improved efficiency and security under a standard assumption in the random oracle and algebraic group models. This new construction significantly outperforms existing UPKE schemes, offering enhanced flexibility and stronger security guarantees. Notably, when applied to the Messaging Layer Security protocol (RFC9420), the new UPKE construction requires less than 2% of the bandwidth of the next-most efficient UPKE implementation, making it a standout advancement in the field.

This work demonstrates the importance of improving UPKE to meet the growing demands of secure multi-party cryptographic protocols.

References: Paper, Slides of the talk